And now to the new terror warning we've heard about tonight. ABC news has learned of signs that terrorists in Syria and Yemen are trying to create explosives that can be smuggled on to a plane as everyday items. They hope passengers with U.S. Passports will be carrying them. ABC's senior justice correspondent Pierre Thomas tells us what he is learning now. Reporter: Intelligence sources tell ABC news that in lawless parts of war-torn Syria, Al qaeda-affiliated terrorists from around the world are now joining forces, to build bombs that are virtually undetectible I airport security. Onto an airplane. One official now calls Syria the new "University" for terrorism training and bomb-making -- intelligence sources say the radicals are trying to create non-metallic bombs that could be hidden in everyday objects, molded into toothpaste, woven into clothing or placed in cosmetics. The fear among intelligence officials is that one of the thousands of foreign fighters in Syria, many of them from western countries, could try to smuggle one of these new generation bombs onto a plane headed for Europe or the U.S. This is the kind of threat that keeps me up at night. Reporter: For instruction, some of the most ruthless elements in Syria have turned to Al Qaeda in Yemen -- proven non innovative bombmakers. They're the group that had the infamous underwear bomb, which slipped onto a u.s.-bound airliner on Christmas day, 2009, but failed to detonate. Since then, the intelligence community believes bombmakers have become even more sophisticated. We're so concerned about it threat to the United States. Still very concerned, as we talked before about Syria as a breeding ground and a staging ground for terrorist groups. Reporter: While no specific plot has been identified, tonight the intelligence community is redoubling its efforts to track foreign fighters moving in and out of Syria. How are confident are American intelligence officials are they that they know how many U.S. Pas passports are there? It's quite an effort to track the margins into Syria, those under home under various levels of investigation and surveillance. The FBI director this week tw pressed concerned that some Americans may be slipping into Syria undetected and that's a real worry. Pierre Thomas, tonight. . We shift to Iraq, the

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